The present invention relates to an apparatus for loading a tubular graft, such as a stent, onto a catheter assembly. Such a catheter assembly can be, for example, of the kind used in typical percutaneous transluminal coronary angioplasty (PTCA) procedures.
In typical PTCA procedures, a guiding catheter is percutaneously introduced into the cardiovascular system of a patient through the brachial or femoral arteries and advanced through the vasculature until the distal end of the guiding catheter is in the ostium. A guide wire and a dilatation catheter having a balloon on the distal end are introduced through the guiding catheter with the guide wire sliding within the dilatation catheter.
The guide wire is first advanced out of the guiding catheter into the patient's coronary vasculature and the dilatation catheter is advanced over the previously advanced guide wire until the dilatation balloon is properly positioned across the arterial lesion. Once in position across the lesion, a flexible and expandable balloon is inflated to a predetermined size with a radiopaque liquid at relatively high pressures to radially compress the atherosclerotic plaque of the lesion against the inside of the artery wall, thereby dilating the lumen of the artery. The balloon is then deflated to a small profile, so that the dilatation catheter can be withdrawn from the patient's vasculature and the blood flow resumed through the dilated artery. As should be appreciated by those skilled in the art, while the above-described procedure is typical, it is not the only method used in angioplasty.
In angioplasty procedures of the kind referenced above, restenosis may occur in the artery, which may require another angioplasty procedure, a surgical bypass operation, or some other method of repairing or strengthening the area. To reduce the likelihood of restenosis and to strengthen the area, an intravascular stent is implanted for maintaining vascular patency. The stent is typically transported through the patient's vasculature where it has a small delivery diameter, and then is expanded to a larger diameter, often by the balloon portion of the catheter. The stent also may be of the self-expanding type.
Since the catheter and stent will be traveling through the patient's vasculature, and probably through the coronary arteries, the stent must have a small, delivery diameter and must be firmly attached to the catheter until the physician is ready to implant it. Thus, the stent must be loaded onto the catheter so that it does not interfere with delivery, and it must not come off of the catheter until it is implanted in the artery.
In conventional procedures where the stent is placed over the balloon portion of the catheter, it is necessary to crimp the stent onto the balloon portion to reduce its diameter and to prevent it from sliding off the catheter when the catheter is advanced through a patient's vasculature. Non-uniform crimping can result in sharp edges being formed along the now uneven surface of the crimped stent. Furthermore, non-uniform stent crimping may not achieve the desired minimal profile for the stent and catheter assembly. Where the stent is not reliably crimped onto the catheter, the stent may slide off the catheter and into the patient's vasculature prematurely and embolize as a loose foreign body, possibly causing thrombosis. Thus, it is important to ensure the proper crimping of a stent onto a catheter in a uniform and reliable manner.
This crimping is sometimes done by hand, which can be unsatisfactory due to the uneven application of force, again resulting in non-uniform crimps. In addition, it is difficult to judge when a uniform and reliable crimp has been applied. Some self-expanding stents are difficult to load by hand onto a delivery device such as a catheter. Furthermore, the more the stent is handled, the higher the likelihood of human error which would be antithetical to crimping the stent properly. Hence, there is a need in the art for a device for reliably crimping a stent onto a catheter.
There have been mechanisms devised for loading a stent on to a catheter. For example, U.S. Pat. No. 5,437,083 to Williams et al. discloses a stent-loading mechanism for loading a stent onto a balloon delivery catheter of the kind typically used in PTCA procedures. The device comprises an arrangement of plates having substantially flat and parallel surfaces that move in rectilinear fashion with respect to each other. A stent carrying catheter can be crimped between the flat surfaces to affix the stent onto the outside of the catheter by relative motion between the plates. The plates have multiple degrees of freedom and may have force-indicating transducers to measure and indicate the force applied to the catheter while crimping of the stent.
Williams et al. also discloses a stent-loading device comprising an elongated tubular member having an open end and a sealed off end. The tubular member houses an elastic bladder which extends longitudinally along the inside of the tubular member. The tubular member and bladder are designed to hold a stent that is to be loaded onto a balloon catheter assembly. Upon placement of the stent over the balloon portion of the catheter, a valve in the loading device is activated to inflate the bladder. The bladder compresses the stent radially inward onto the balloon portion of the catheter to a reduced diameter to thus achieve a snug fit.
Although the above-described methods by which stents are crimped are simple, there is a potential for not crimping the stent sufficiently tight to prevent it from loosening in the tortuous anatomy of the coronary arteries. Because the amount of compression needed to be applied by the fingers will vary with the (a) strength of the operator, (b) day-to-day operation, (c) catheter and balloon material and configuration, (d) experience of the operator in crimping, and (e) other factors, the tightness in which the stent is crimped onto a balloon catheter may vary considerably.
Indeed, because of these factors, the tightness follows a normal or Chi square distribution. At the lower tail end of the distribution, the stents will be loose and susceptible to movement on the balloon during insertion. At the higher tail end, the stent will be too tight and will affect the expansion characteristics (i.e., a dog bone effect) of the balloon.
Currently, a majority of stents are crimped onto the balloons of PTCA delivery catheters by deformation of the stent. In these cases, there is no adhesive on the balloons. As a result, a one to three percent stent loss has been observed. If the stent detaches from the balloon, the patient may require surgery to retrieve the stent.
To minimizes the stent loss problem, some manufacturers premount or precrimp their stents onto the PTCA balloons to ensure that the stents are securely attached to the catheter. This is an extra cost to the manufacturer, and does not give the cardiologist the choice to use any type of PTCA delivery catheters. It is also an added cost to the cath lab which pays for the extra PTCA delivery catheter.
One solution has been to coat the PTCA balloons with a pressure sensitive adhesive. The adhesive anchors the stent onto the balloon of the PTCA delivery catheter. Stent loss during stenting is reduced as a result of this approach.
However, several problems arise when it is necessary to remove the balloon from the stented artery at the deployment site. First, stent apposition against the arterial wall may be affected as the catheter balloon is deflated after stent expansion. Second, as the balloon is deflated, the stent struts which are still adhered to the balloon retract with the balloon. As a result, the struts can be bent, deflected, or deformed towards the lumen of the artery. This may result in deleterious effects on the blood compatibility of the stent and affect the stent's ability to support the artery.
Third, the stent can be displaced from the lesion as the catheter is pulled out of the patent. This leads to mispositioning of the stent with respect to the lesion.
The stent is no longer supporting a lesion as it was intended, but after it has shifted, is supporting a healthy portion of the artery.
Fourth, residual adhesives may be transferred onto the stent's inner surface as the balloon is physically peeled away from the stent. The presence of the residual adhesive on the metal surface may affect crossability of other catheters and guidewires. Residual adhesives left on the stent may affect the blood compatibility of the stent as well.
Another approach is disclosed in U.S. Pat. No. 5,100,429 to Sinofsky et al. This approach suggests anchoring an endovascular stent to a balloon catheter with a photodegradable adhesive. Once the stent is delivered and expanded against the arterial wall, light is directed onto the adhesive resulting in degradation of the adhesive.
A problem with using a photodegradable adhesive is that it may be released into the blood stream and tissue when the adhesive breaks down in the presence of UV light. There is also an added engineering requirement to integrate an optical fiber into the delivery catheter or some other means to expose the adhesive to light. Without this light source, the adhesive is not degraded and the stent cannot be detached from the PTCA balloon after stent expansion. Including an optical fiber to the delivery catheter not only increases costs to the manufacturer, but also makes the delivery catheter profile larger and ungainly.
In view of the foregoing, there is a need for a catheter having a facility to secure a stent thereon, yet easily releases the stent from the catheter on command at the deployment site.